1. Field of the Invention
The invention relates to a complete and reproducible process for obtaining new strains of yeasts for bread making. It also relates to the novel strains of yeast thus prepared as well as to fresh or dried yeast for bread making prepared from said new strains.
2. Description of the Prior Art
Quick strains of yeasts are already known which are adapted to maltose--that is to say, enabling the preparation of yeasts which release a large amount of CO.sub.2 with doughs constituted by flour and water--and which remain active with doughs of little sweetness, that is to say, not containing more than 5% by weight of sugar with respect to the flour, namely less than 3.3% by weight with respect to the dough.
It is found that these strains exhibit performances which drop appreciably when the sugar content of the dough increases and notably when it exceeds 10% by weight with respect to the flour. Now, such doughs represent a non-negligeable part of bread-making in certain countries.
Moreover, these strains, whose use has been generalised in yeast making for about 10 years, are rapdily inhibited as soon as the dough contains significant concentrations of acetic, sorbic or propionic acids or their salts.
Acid or sour doughs occur in the manufacture of rye bread, leavened bread and others, whose acidity, corresponding to a pH below 4.7, is contributed by a mixture of about 10-50% by weight of acetic acid and about 50 to 90% by weight of lactic acid and which are rarely sweetened, represent also a non-negligeable part of bread making.
Finally, in all countries, there is added to the products of bread making intended to have a long period of preservation or a preservation under difficult conditions, mould-inhibiting agents such as acetic, sorbic, propionic acids and their salts, and this whatever the proportion of sugar contained in the dough. It is known that a bread making yeast resistant to acetic acid, that is to say, whose fermenting power is not inhibited significantly in the presence of undissociated acetic acid, has generally the same property, that is to say, a better resistance, with respect to inhibiting doses of undissociated propionic or sorbic acid.
To overcome the inadequacies of the prior art with regard to bread-making yeast strains, it is an object of the invention to provide a process adapted to permit the obtaining, in simple and reproducible manner, of novel strains of yeast adapted to maltose, characterised by the fact that the yeast, both fresh and dried, of which they enable the preparation and of which certain at least constitute novel industrial products, are:
either still better adapted to maltose,
or active with sweetened doughs, that is to say with dough containing at least 5% by weight of sugar with respect to the flour,
or active with acid doughs,
or endowed, preferably, with two of the three above-mentioned properties and preferably with all three.
In order to do this, the specialist in the field has the choice between:
on the one hand, modifying the processes of propagation of yeast, that is to say, its processes of cultivation and,
on the other hand, obtaining novel strains by mutation and/or hybridation.
Modification of the cultivation processes is laborious and difficult to put into practice and often magnifies one property more or less to the detriment of another.
Research for novel strains by hybridation and mutation poses complex problems. It is highly uncertain if the objectives and the phenomena in play are not well mastered and if the crossing plans or the mutation process are not clearly defined. It results in any case in the obligation to test thousands even tens of thousands of colonies, which is impossible in practice by means of tests with release of gas under specific conditions (flour, sugar or organic acid mediums), said tests requiring cultivation in a fermenter of some liters as described in Example 1 of French patent application No. 75 20943, the harvesting of this yeast and at least five measurements of gas release according to tests of type A which will be considered below. The problem is complicated by the fact that the results obtained are reproducible with difficulty; for example, a slight modification, difficult to master in the conditions of cultivation can result in considerable variations in respect to the criteria measured.
Nevertheless, research for novel strains is theoretically the best solution, all the more as the employment of specific cultivation conditions can only improve, reinforce the natural properties possessed by the hybrids or mutants.
The two routes of research are in fact complementary and not concurrent.